void
__shared_weak_count::__release_weak() _NOEXCEPT
{
    // NOTE: The acquire load here is an optimization of the very
    // common case where a shared pointer is being destructed while
    // having no other contended references.
    //
    // BENEFIT: We avoid expensive atomic stores like XADD and STREX
    // in a common case.  Those instructions are slow and do nasty
    // things to caches.
    //
    // IS THIS SAFE?  Yes.  During weak destruction, if we see that we
    // are the last reference, we know that no-one else is accessing
    // us. If someone were accessing us, then they would be doing so
    // while the last shared / weak_ptr was being destructed, and
    // that's undefined anyway.
    //
    // If we see anything other than a 0, then we have possible
    // contention, and need to use an atomicrmw primitive.
    // The same arguments don't apply for increment, where it is legal
    // (though inadvisable) to share shared_ptr references between
    // threads, and have them all get copied at once.  The argument
    // also doesn't apply for __release_shared, because an outstanding
    // weak_ptr::lock() could read / modify the shared count.
    if (__libcpp_atomic_load(&__shared_weak_owners_, _AO_Acquire) == 0)
    {
        // no need to do this store, because we are about
        // to destroy everything.
        //__libcpp_atomic_store(&__shared_weak_owners_, -1, _AO_Release);
        __on_zero_shared_weak();
    }
    else if (__libcpp_atomic_refcount_decrement(__shared_weak_owners_) == -1)
        __on_zero_shared_weak();
}